Since the invention of elevators approximately 125 years ago, both passenger and cargo elevators have been built within the following three categories: The first one, which continues to be the one most often used, is that of an elevator equipped with metal cables and electric motor systems. The second (with height limitations) is that of elevators that use hydraulic pistons, be they simple or telescopic pistons; and the third one (with greater restrictions in length of run) are those that use screws in either a direct or an indirect manner. Each one of these elevators has specific applications where their use is recommended. The first two categories can have variances of use with counterweights which significantly reduce the size of the motors and make them more efficient.
In the case of elevators equipped with traction cables, the counterweight is a very important part and it generally represents 60% of the weight of the cabin, since heavier counterweights would cause stability problems during the braking process as they are used in open elastic loops. This means that they only connect the cabin and the counterweight on the upper side of the cabin. This demands that the cabin design must have a greater inertia than that of the counterweight to avoid tugs during the process of braking. In the case of this invention, the elevator substitutes the traction cables by metal chains; in the same manner it also substitutes the traction pulleys by sprockets, but in addition it does this by means of a closed loop, which is both above and below, and by this means it ensures greater stability of the traction system.
Cable elevators have the problem that the cables stretch approximately 2% of their length. This stretching is inherent to steel cables and to the formation of the twisting of the cables (wire strands) which, when being tensed, will temporarily thin out the section of the cable, but with a tendency to permanent deformation. The progressive stretching of the cables, along with their folding on the traction pulley and on the deflecting pulley, originate cable fatigue as a result of which very high safety factors have to be used (10 to 1). In the same manner, traction pulleys have multiple grooves with the shape of the wire rope to ensure greater traction and avoid slipperiness. Nevertheless, these grooves are the result of the shape of the outstretched cable so that, when the cable has given way, it becomes a friction element causing wear between the cable and the pulley.
The constant stretching of the traction cables results in misalignments in the floor stops of the elevator thereby creating greater maintenance requirements.
The traction system hereby proposed allows the use of very heavy counterweights without creating instability during the braking process as a result of the fact that this is an inelastic closed loop. This allows a better balance between the weight of the cabin and the weight of the counterweight. In addition, it allows us to increase the counterweight up to 50% over and above the load that is to be vertically carried. This then requires less electric power to reach movement at the required speed.
In general, the traction elements of cable elevators consist of electric motors coupled with helicoidal speed reducers. These slow up the speed of the motor and increase the torque in the outgoing shaft that couples with the traction pulley. Due to the nature of the design and manufacturing process of these speed reducers, they have efficiency levels of around 80% with progressive wear since they operate through the friction of a pinion against a crown. This type of speed reducers also requires constant maintenance to avoid increasing friction coefficients to a very high level.
Elevator motors are normally electric, be they direct or alternating current, and generally in two speeds. Nowadays in elevators for great heights variable frequency motors are used to provide smoother start-ups and stops through the use of an inverter. The elevator that is the subject of the present invention uses one, two or up to four servomotors coupled to planetary-type speed reducers. These in turn spin the traction sprockets that make the traction chains move raising or lowering both the elevator cabin and the counterweight. The use of servomotors carries the benefit that we are using pre-programmable motors which have improved electrical and mechanical features for frequent starts and stops, they are compact, of variable speeds, perfectly precise, the number of turns at which they must spin can be programmed, as well as the acceleration and deceleration time or distance, maximum torque, they are reversible, have dynamic brakes and provide us with feedback of the whole of the motor's behavior by means of its servo-amplifier and encoder.
Traditional elevators are controlled by means of integrated circuits with microprocessors which receive the signals from sensors of the inductive type or micro switches that define calls or relative positions of the elevator cabin. The integrated circuits are programmed to perform the operating sequences that consist in rising, lowering (with the application of two speeds or variable speeds), and re-leveling, opening and closing doors. The elevator that is the subject of this invention modifies the control system by adopting the advantages inherent to servomotors. These, because they are intelligent motors, have already integrated the encoders and servo amplifiers which provide directly to the servomotors the start-up, acceleration, operating speed, and number of turns their shafts must perform, the programmed torque, the deceleration and stop point, in addition to obtaining a feedback of the exact behavior and the status or final position of the servomotor. Therefore, in this case there is no need for external sensors, since the whole control system of the servomotors is intrinsic to them. To control sequential movements, such as opening and closing doors, as well as calls to the elevator during its trip upwards or downwards, one uses a “programmable logic controller” (PLC) to process digital or analogical signals that can be fed into the programmable control logic with a very high level of confidence and simplicity in the program.